1. Field of the Invention
This invention relates generally to a corona discharge ignition system, and more particularly to methods for controlling energy supplied to the corona igniter system.
2. Related Art
Corona discharge ignition systems provide an alternating voltage and current, reversing high and low potential electrodes in rapid succession which enhances the formation of corona discharge and minimizes the opportunity for arc formation. The system includes a corona igniter with a central electrode charged to a high radio frequency voltage potential and creating a strong radio frequency electric field in a combustion chamber. The electric field causes a portion of a mixture of fuel and air in the combustion chamber to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture, which is referred to as an ignition event. The electric field is preferably controlled so that the fuel-air mixture maintains dielectric properties and corona discharge occurs, also referred to as a non-thermal plasma. The ionized portion of the fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining portion of the fuel-air mixture. Preferably, the electric field is controlled so that the fuel-air mixture does not lose all dielectric properties, which would create a thermal plasma and an electric arc between the electrode and grounded cylinder walls, piston, metal shell, or other portion of the igniter.
In addition, preferred corona discharge ignition systems operate such that the corona igniter is driven at its resonant frequency, because resonant frequency operation allows the corona igniter to provide a high output and efficiency. However, accurately controlling the drive frequency of the corona igniter to be equal or close to the resonant frequency presents challenges; especially since the design of the corona igniter is constantly developing and improving, leading to changes in the resonant frequency. For example, one recently developed method used in effort to achieve resonant frequency operation require multiple cycles in order to achieve a lock to the correct frequency and is unable to accurately follow rapid frequency changes. Another method allows operation over only a limited range of frequencies. A third method utilizes a programmable digital or mixed-signal controller to control switches of the system with suitable timing and accuracy, but this type of controller requires complex specifications leading to high overhead and thus high costs.